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Re: [Qemu-devel] [RFC 0/9] Introduce light weight PC platform pc-lite

From: Chao Peng
Subject: Re: [Qemu-devel] [RFC 0/9] Introduce light weight PC platform pc-lite
Date: Tue, 21 Jun 2016 09:23:08 +0800
User-agent: Mutt/1.5.23 (2014-03-12)

> It would be nice to at least see the patches. :)
> I think a lightweight q35 platform that can run the usual firmware could
> be acceptable in QEMU.

OK, I will send out v2.

> >> 2) this:
> >>
> >>> - it loads guest kernel directly, no BIOS, no bootloader, no realmode
> >>>   code;
> >>
> >> ... which is related to Linux-only support.  How much does this gain
> >> over a minimal firmware (either SeaBIOS with the fw_cfg DMA interface,
> >> or qboot with cbfs in parallel flash)?
> > 
> > We have tried Q35 version (as described above) with both SeaBIOS and qboot.
> > The 'perfect' time with optimized BIOS we have seen is ~15ms, with the
> > additional time in kernel real mode code, the total time overhead comparing
> > to current Linux-aware implementation is more than 40ms. This sounds still
> > a little too much for us.
> I guess it is related to real mode decompression code?

Yes, that's the major part.

> My main issue is that there are other things that the firmware does.
> Not all of them are necessary (e.g. SMRAM is not needed, most PCI
> devices need not be initialized), but in general we don't like putting
> code in QEMU that modifies the guest state.  For example another Intel
> person is adding code to SeaBIOS that initializes the feature control MSR.
> I wonder if Linux could run as a multiboot-compliant ELF file, and what
> the performance would be...  Multiboot omits the real mode stub.

I can look into this. Thanks.


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